The activity of the aryl hydrocarbon hydroxylase of rat liver and of cultured rat liver cells is stimulated by pre-exposure of the rat or cultured cells to particular carcinogens. It is the purpose of this project to investigate the mechanisms by which this induction occurs. As the first stage of this investigation, electrophoretic estimations of the levels of the various different species of the cytochrome P-450 moiety of the hydroxylase system are being made in rat liver, after treatment of the rats with various enzyme inducers.